1. Field of the Invention
The present invention relates to a piezoelectric oscillating element having three terminals, and also to a method for making such a 3-terminal oscillating element. It further relates to an oscillator employing such an oscillating element.
2. Description of the Prior Art
In compliance with the recent development of microcomputer, a piezoelectric oscillating element made of ceramic has become widely used in a Colpitts circuit. Particularly, in a C-MOS type LSI, a circuit shown in FIG. 1 is often employed. In FIG. 1, IV designates an inverter, R designates a feedback resistor, CR.sub.1 designates a 2-terminal piezoelectric oscillating element made of ceramic, and C.sub.1 and C.sub.2 designate, respectively, externally provided capacitors. In order to obtain a stable oscillation, an input voltage V.sub.1 and an output voltage V.sub.2 are set to appropriate voltages by the adjustment of capacitance of each of the capacitors C.sub.1 and C.sub.2.
The circuit shown in FIG. 1, however, takes disadvantages in that (i) since the temperature characteristic of piezoelectric ceramic element CR.sub.1 differs from that of the externally provided capacitors C.sub.1 and C.sub.2, the temperature characteristic of oscillating frequency becomes deteriorated, and that (ii) the circuit requires two externally provided capacitors resulting in bulky in size.
In the recent developments, these two disadvantages have been solved. A development for eliminating the disadvantage (i) is described below.
In FIG. 2, there is shown a circuit which is equivalent to the circuit shown in FIG. 1. In the circuit of FIG. 2, the oscillating frequency f.sub.osc can be given as: ##EQU3## An anti-resonant frequency f.sub.a of the piezoelectric oscillating element CR.sub.1 solely can be given as: ##EQU4## Generally, a temperature characteristic of a resonant circuit with a resonant frequency f.sub.0 =1/2.pi..sqroot.LC can be given as: ##EQU5## wherein t is temperature. From the above equation, an excellent temperature characteristic of resonant frequency f.sub.0 can be obtained when ##EQU6##
In usual cases, a material having an excellent temperature characteristic of anti-resonant frequency f.sub.a is employed as the piezoelectric oscillating element CR.sub.1. In such a material, ##EQU7## Therefore, when the temperature coefficient of each of the capacitors C.sub.1 and C.sub.2 is fixed to an amount equal to that of an equivalent capacitor C0 in the equivalent circuit, the resultant is that: ##EQU8## This can be done by employing the same material for the capacitors C.sub.1 and C.sub.2 and for the piezoelectric oscillating element CR.sub.1. When the above equation is accomplished, an excellent temperature characteristic of oscillating frequency can be obtained, and thus eliminating the above described disadvantage (i). A further detail of the above arrangement is disclosed in British Pat. No. 1,576,704.
A development for eliminating the disadvantage (ii) is disclosed in U.S. Pat. No. 4,336,510 in which a possibility of eliminating the externally provided capacitors C.sub.1 and C.sub.2 is disclosed. According to U.S. Pat. No. 4,336,510, a 3-terminal oscillating element CR.sub.2 as shown in FIG. 3 is employed. An example of one embodied form of the 3-terminal oscillating element CR.sub.2 is shown in FIGS. 4 to 6, which comprises a rectangular, such as a square piezoelectric ceramic plate 1 having opposite flat major surfaces. The upper flat major surface, as shown in FIG. 4, is provided with two electrodes, which are center electrode 2 in a shape of square and rim electrode 3 surrounding the center electrode 2. The lower flat major surface, as shown in FIG. 5, is provided with a common electrode 4 entirely on the lower major surface. FIG. 6 shows the direction of polarization in the piezoelectric plate 1 in which a region between center electrode 2 and common electrode 4 and a region between rim electrode 3 and common electrode 4 are poled in opposite directions. When the piezoelectric plate 1 is poled in opposite directions as shown in FIG. 6, the transformation ratio n of an ideal transformer, which will be described in detail later in connection with FIGS. 8 and 9, will become greater than zero and, therefore, the phase difference between the input voltage and output voltage of the oscillating element CR.sub.2 will be 180.degree., resulting in oscillation of the element CR.sub. 2 in an expansion mode. In this case, the element CR.sub.2 vibrates in a single mode.
Although U.S. Pat. No. 4,336,510 teaches the possibility of use of 3-terminal oscillating element CR.sub.2 in place of the combination of 2-terminal oscillating element CR.sub.1 and externally provided capacitors C.sub.1 and C.sub.2, it does not teach the necessary condition the 3-terminal oscillating element CR.sub.2 must meet to accomplish the equivalency with the combination of 2-terminal oscillating element CR.sub.1 and externally provided capacitors C.sub.1 and C.sub.2.